Align neck body to head geometry
1- Enable the Snapping Tool clicking the magnet on the 3D view bar. 
2- Next to the magnet icon hit the icon next to it to select snap to what> Select 
Verex.
3- Select a vertex along the neck edge. 
4- Move cursor close to its desired destination.
5- Hit G, then ENTER when in snapped position. 

Note: Transform manipulators doesn't work well with the magnet.